The present invention relates to subsea wellbore intervention operations and, in particular, to methods and systems for tracking a toolstring at subsea depths.
In the oil and gas industry, subsea operations include systems used to explore, drill, harvest, and/or maintain subterranean hydrocarbon formations located under the ocean floor. Access to the subterranean formations is usually via one or more wellhead installations arranged on the seabed and providing access to a corresponding wellbore drilling into the subterranean formation. Equipment used in subsea operations is generally constructed to withstand extreme conditions, be economically cost-effective, and safe guard the oceanic environment. Conventional systems used in subsea operations include a service vessel or rig located at the surface and from which a toolstring is lowered to the seabed in order to undertake one or more intervention operations on the wellhead installation. The toolstring is usually brought into proximity of the wellhead installation using a remote operated vehicle (ROV) remotely operated from a support vessel also located at the surface. The ROV is usually tethered to the support vessel and an operator residing on the support vessel is able to navigate or otherwise manipulate the general position of the ROV in the open water.
In typical operation, the ROV is brought to the surface and is attached to the toolstring in order to transport the toolstring from the surface and to the wellhead installation. Some ROVs use a light source in order to see the toolstring. In this regard, the toolstring is typically painted black which improves light reflection in the subsea environment. Once arriving at the wellhead, the ROV can be controlled to have the toolstring perform repair and intervention tasks on the wellhead or otherwise stab the toolstring into the wellhead for downhole operations. Once the intended subsea operation is completed, the ROV typically transports or otherwise follows the toolstring back up to the ocean surface where a tool changeout can occur on the toolstring. Once the toolstring is again ready to be introduced into the subsea environment, the ROV is again attached thereto and the diving and locating process is repeated.
Accordingly, the ROV must typically dive and surface with the toolstring each time a new tool or tool changeout is required on the toolstring. This drastically increases the time required to perform the intended subsea operations, thereby increasing the related costs of operating the surface equipment (i.e., the service rig, the support vessel, etc.). To reduce operation time, some systems include a carousel of tools arranged on the toolstring so that the ROV can undertake a variety of subsea operations and interventions from a single trip to the wellhead. However, if the scope of subsea operations changes, the carousel of tools must surface along with the ROV operating therewith as previously discussed.
In current systems, toolstring turnaround times for subsea operations are limited by the speed of the ROV. For example, while toolstrings can be deployed in subsea operations at a line speed of 200-300 ft/min, the speed at which an ROV can dive with the toolstring is about 75 ft/min. As a result, the ROV is the weak link of the operation in terms of time consumption. Accordingly, there is a need in the art to more quickly and efficiently perform subsea operations such as by eliminating the need for the ROV to dive and surface with the toolstring. Moreover, there is a need in the art for a system which detects the location of a toolstring while at subsea depths.